1. Field of the Disclosure
The disclosure relates to a doctor blade dosing system for a device used for coating material webs, particularly paper or paperboard, that has a doctor blade rod which serves as a dosing element while being held inside a slot of doctor blade bed that is mounted in a support that can be fastened inside the frame of the system.
2. Related Technology
It is a known procedure to coat continuous paper or paperboard webs by means of devices that have an application system for applying an excess of liquid coating material and a downstream dosing system fitted with a doctor blade rod as the dosing element that scrapes off the excess coating material until the desired application weight is achieved. The application and dosing procedures take place either directly onto the web or indirectly, first of all onto a roller that deflects the web and subsequently transfers the dosed film of coating material onto the web.
A doctor blade dosing system of the generic type is described in DE-A 3022955. The doctor blade rod is held in a doctor blade bed made of rubber-elastic material that is mounted in a support that is connected to the machine frame. At its rear side facing away from the doctor blade rod, the doctor blade bed is supported by a pressure tube so that the thickness of the dosed film on the roller or on the web can be varied to a certain extent by means of the pressure in the pressure tube. In the doctor blade bed, there are two open rinsing channels that run parallel to the doctor blade and that are open towards the doctor blade rod, into which a rinsing liquid, for example, water can be fed in order to lubricate the bearings of the doctor blade rod and to remove coating material that has penetrated there. Such doctor blade dosing systems in which the doctor blade bed is supported by a pressure tube are called roll doctor systems. They are used for direct dosing onto the web in an area where they are supported by a mating roll. So-called revolving doctor systems are likewise known in which the web is conveyed around the doctor blade in a free-running area. With these systems, the doctor blade bed is secured in a rigid holder without a pressure hose.
The doctor blade rods are normally made of steel and have a diameter between 6 mm and 40 mm. Since the pigment coatings normally used as the coating material for coating paper or paperboard are highly abrasive, it is a known procedure to coat the surface of the doctor blade rods with wear-resistant materials in order to increase the wear-resistance. Depending on the application purpose, doctor blade rods having a smooth lateral curved surface or doctor blade rods with circumferential grooves are used. During the operation of doctor blades with a smooth lateral curved surface, a narrow gap exists relative to the web to be coated so that the desired amount of coating material passes through this gap. Doctor blades with circumferential grooves are in contact with the web or roll during the dosing. The application weight that remains is determined primarily by the groove cross section through which the coating material can pass.
In order to hold the doctor blade rod securely in the slot, the known doctor blade beds have to be manufactured with precise dimensions. Since the doctor blade beds have a relatively large cross section, they are manufactured in a casting process in order to achieve the necessary dimensional accuracy and/or the required dimensional accuracy is attained by means of metal-cutting. The production is material-intensive and labor-intensive.
The doctor blade rod, which rotates during operation, causes considerable wear and tear to the doctor blade bed. Consequently, the material-intensive and labor-intensive doctor blade beds have to be replaced at regular intervals. Such replacement entails considerable costs. In order to reduce the replacement costs due to wear and tear without impairing the quality of the coating, the roll doctor system according to German Patent Application No. 10045515 has a support with a slot into which the doctor blade bed can be placed with the inserted doctor blade rod. The doctor blade bed is configured as the smallest possible wearing part.